The change to Google's algorithms is aimed at helping websites with original content and information rank higher. The action marks a rare admission of fallibility by the search giant.

Google has come under attack over the quality of its search results. One… (Andrew Harrer, Bloomberg )

Reporting from San Francisco — Google Inc., under fire for letting some websites manipulate its powerful search engine, has made a major change to the way it ranks results.

The change to Google's algorithms, which it employs to help users find what they want when they search the Web, is aimed at helping websites with original content and information, including in-depth reports or "thoughtful" analysis, rank higher. The change affects about 12% of queries, Google said.

Making such a significant change to the search engine in such a public way marks a rare admission of fallibility by Google, which handles about two-thirds of the world's search queries and generates most of its sales from search advertising.

Despite the dominance of its search engine, Google has taken hits in recent months for allowing sites to rise in search results that deliver questionable or little value to Web searchers. Vivek Wadhwa, a visiting scholar at UC Berkeley's School of Information, described Google in a blog post as a "tropical paradise for spammers and marketers."

The debate over the quality of Google's search results has not shrunk its commanding lead over Microsoft Corp.'s Bing search engine. But Google is taking steps to protect its public image as that debate spills into the mainstream from the more arcane world of search optimization, where professionals make a living helping websites rise in Google rankings.

Google has benefited for years from the assumption that its search results are "better than anyone else's," said Danny Sullivan, editor in chief of Search Engine Land, an influential technology blog. "There is no doubt that Google's relevancy has come under attack in an unprecedented way in recent months and it has been snowballing. This is an effort to slow down that snowballing."

Google recently introduced an extension for the Google Chrome Web browser that lets users manually block sites from appearing in their results. The effort is similar to that of a new Google competitor, start-up Blekko, which blocks sites that users mark as spam.

Google did not divulge which websites would drop in its rankings, but many search-optimization professionals have speculated that Google is targeting Demand Media's eHow and other so-called content farms.

Google is trying to deliver "more substantial and authoritative answers than the short summaries from websites that have built their businesses on giving bites and chunks of information," Sullivan said. Matt Cutts, who leads Google's spam-fighting efforts, has said in recent weeks that Google was working on tweaks to its algorithms to fix the problem.

Demand Media, based in Santa Monica, said in a blog post that it had not seen a "material net impact," but its shares traded lower Friday on the news. Andrew Shotland, who runs Local SEO Guide, a consulting firm that advises large websites, said some of his clients were "definitely seeing a negative impact."

"Today is not different than any other day: I am telling clients they need more original and better content on their pages, it's just now it's more important than it was a few weeks ago," Shotland said.

The stakes for sites that rely on Web traffic from Google are high. Even a slight dip in traffic can slash revenue. One search-optimization professional said some sites could see drops of as much as 30%. Another who works for some of the world's largest websites said significant drops in traffic would result in "a real human cost," as the affected sites lay off workers.

Jonah Stein, founder of Its the ROI, a San Francisco search-engine optimization and marketing company, said he turned to Google six months ago when his wife tore her rotator cuff. Instead of getting top results from medical sites, he was directed to what he called "garbage" content farms. The same search Friday yielded orthopedic and sports injury sites as top results.

"So much energy has gone into trying to make money online that anywhere you dig a little, you are going to find websites delivering less value than they should," Stein said. "The sad thing is that the sites that are doing a really great job of delivering content and value are not the ones rising to the top."

"Helping put those sites at the top of search results is core to what a search engine should be striving to do," he said. But with so many marketers gaming the system, he said, "it's not clear that anybody knows how to do that at this point."